1. Field of the Invention
This invention generally relates to an apparatus for driving an electric load or a current consumer. This invention particularly relates to an apparatus for driving an electric load such as a coil, an inductor, or a lamp whose internal resistance varies as it is energized.
2. Description of the Related Art
As an electric load such as a coil or a lamp is energized, it is heated and hence its internal resistance increases. Generally, a switching element is used to turn on and off the electric load. It is known to use a switching element or a current adjusting device to control the magnitude of a current flowing through the electric load. At the start of the energization of the electric load, the internal resistance of the electric load is low and thus a great current tends to flow therethrough. Such a great current is referred to as a turn-on inrush surge current. In some cases, the turn-on inrush surge current damages the switching element or the current adjusting device.
U.S. Pat. No. 4,987,348 corresponding to Japanese published unexamined patent application 3-256407 discloses a bilevel current limiting control circuit which enables a semiconductor controlled power switch with a limited power handling capability to operate near the inside border of the safe operating area (SOA) characteristic of the power switch. The control circuit is especially useful in limiting the turn-on inrush surge current to an incandescent lamp load supplied via the semiconductor power switch. The control circuit includes a series circuit made up of the power switch, a current sensing resistor and a lamp load connected to the terminals of a source of supply voltage. First and second comparison circuits, arranged to provide first and second different current limits for the control circuit, are connected to the sensing resistor. A third comparison circuit compares the load voltage with a reference voltage to control a selector device that selects the output of the first or second comparison circuit as a function of the level of the load voltage. The output of the selected comparison circuit is applied via a feedback circuit to a control electrode of the power switch so as to control the load current to the first and second current limit values as selected in response to the load voltage.
In the control circuit of U.S. Pat. No. 4,987,348, the load current is controlled to the first and second current limit values. When the lamp load is turned on, the load current abruptly increases to the upper limit current value. Such an abrupt increase in the load current tends to cause spike noise, that is, high-frequency noise. When the lamp load is turned off, the load current abruptly drops to a null level. Such an abrupt drop in the load current tends to cause spike noise (high-frequency noise). The spike noise may cause wrong operation of another control apparatus if it is propagated thereto.